Field
The disclosed concept pertains generally to vacuum circuit breakers and other types of vacuum switchgear and related components, such as vacuum interrupters and shield walls. In particular, the disclosed concept pertains to axially positioning a pair of separable contact assemblies located in a vacuum envelope of a vacuum interrupter employing a floating center shield component composed of copper-chromium alloy-based material, such that the contact gap between the opposing contact surfaces of the assemblies aligns with a portion of the shield wall having a maximum thickness and outer diameter.
Background Information
Vacuum interrupters are typically used to interrupt high voltage AC currents. The interrupters include a generally cylindrical vacuum envelope surrounding a pair of coaxially aligned separable contact assemblies having opposing contact surfaces. The contact surfaces abut one another in a closed circuit position and are separated to open the circuit. Each electrode assembly is connected to a current carrying terminal post extending outside the vacuum envelope and connecting to the external circuit.
An arc is typically formed between the contact surfaces when the contacts are moved apart to the open circuit position. The arcing continues until the current is interrupted. Metal from the contacts that is vaporized by the arc forms a neutral plasma during arcing and condenses back onto the contacts and also onto a vapor shield placed between the contact assemblies and the vacuum envelope.
The vacuum envelope of the interrupter typically includes a ceramic tubular insulating casing with a metal end cap or seal covering each end. The electrodes of the vacuum interrupter extend through the end caps into the vacuum envelope.
Vacuum interrupters are key components of vacuum-type switchgear. It is typical for interrupters for vacuum-type circuit breakers using transverse magnetic field contacts to include a tubular center shield to protect the internal wall of the tubular insulating casing from being coated with the metallic product of the burning of the arc on the contacts. The tubular center shield can be mounted and electrically connected to either one end of the metallic construction of the vacuum interrupter; in this case the center shield is called fixed. Alternatively the center shield can be mounted, via a center flange, to the tubular insulating casing and electrically insulated from either of the metallic ends of the vacuum interrupter; in this construction the center shield is called floating. The center shield can be an assembly of multiple components. For example, U.S. Pat. No. 4,020,304 prescribes a center shield assembly consisting of a middle portion made out of copper and two end portions made out of stainless steel.
As prescribed in U.S. Pat. No. 4,553,007, it is advantageous for the arcing portion of the tubular center shield, that is, the portion of the center shield surrounding the contact gap, to be made out of a material comprised of the same two metallic components as the separable metallic electric contacts, which for all practical purpose are copper and chromium. The employment of a center shield with the arcing portion made out of copper-chromium alloy material allows a close proximity of the shield to the contacts, as such a shield is capable of enduring not only the unintentional bowing out to the shield of the burning arc in between the two separating contacts, but also intentional participation and sharing of the arcing duty required to interrupt a high current. For that reason, center shields with the arcing portion made out of copper-chromium (Cu—Cr) alloy-based material are often used in vacuum interrupters for the highest fault current ratings, especially those of the transverse or radial magnetic field type.
FIG. 1 is a cross-section view of a vacuum interrupter 10 in accordance with the prior art, which employs a center shield component 24 made out of arc-enduring Cu—Cr alloy-based material. FIG. 1 shows a cylindrical insulating tube 12, consisting of two cylindrical pieces which, in combination with end seals 51 and 52, forms a vacuum envelope 50. The center shield component 24 is secured to the insulating tube 12 by a center flange 25 that is typically braze-joined. The center shield component 24 surrounds a first electrode assembly 20 and a second electrode assembly 22 to prevent metal vapor from collecting on the insulating tube 12, and to prevent an arc from hitting the insulating tube 12. The insulating tube 12 is preferably made of a ceramic material such as alumina, zirconia or other oxide ceramics, but may also be glass. The Cu—Cr alloy-based center shield component 24 is the middle portion of a center shield assembly, which also includes opposing metal end components 13, 15. Overlaps 37, 38 are formed by a metal portion of the end components 13, 15, respectively, overlapping a portion of the Cu—Cr alloy-based center shield component 24. The first and second electrode assemblies 20 and 22, respectively, are axially aligned within the vacuum envelope 50. The first electrode assembly 20 includes a bellows 28, a bellows shield 48, a first electrode contact 30, a first terminal post 31, and a first vapor shield 32. The second electrode assembly 22 includes a second electrode contact 34, a second terminal post 35, a second vapor shield 36, and an end shield 58. While the vacuum envelope 50 shown in FIG. 1 is part of the vacuum interrupter 10, it is to be understood that the term “vacuum envelope” as used herein is intended to include any sealed component having a ceramic to metal seal which forms a substantially gas-tight enclosure. Such sealed enclosures may be maintained at sub-atmospheric, atmospheric or super-atmospheric pressures during operation.
The first and second electrode assemblies 20 and 22, respectively, are axially movable with respect to each other for opening and closing the AC circuit. The bellows 28 mounted on the first electrode assembly 20 seal the interior of the vacuum envelope 50 formed by the insulating tube 12 and end seals 51 and 52, while permitting movement of the first electrode assembly 20 from a closed position as to an open circuit position (as shown in FIG. 1). The first electrode contact 30 is connected to the generally round first terminal post 31 which extends out of the vacuum envelope 50 through a hole in the end seal 51. The first vapor shield 32 and the bellows shield 48 are mounted on the first terminal post 31 in order to keep metal vapor off the bellows 28 and the insulating tube 12. Likewise, the second electrode contact 34 is connected to the generally round second terminal post 35 which extends through the end seal 52. The second vapor shield 36 and the end shield 58 are mounted on the second terminal post 35 to protect the insulating tube 12 from metal vapor. The second terminal post 35 is rigidly and hermetically sealed to the end seal 52 by means such as, but not limited to, welding or brazing. The center shield component 24 is not electrically connected to, and hence is electrically floating from, either the first or the second electrode assemblies 20 and 22.
FIG. 1A is a detail view of the vacuum interrupter 10 and the center shield assembly consisting of the arc-enduring Cu—Cr alloy-based center shield component 24 and, opposing metal end components 13, 15 shown in FIG. 1, when the vacuum interrupter 10 is in an open position, with an axial contact gap 14 formed between the surfaces of the first and second electrode contacts 30, 34 of the first and second electrode assemblies 20, 22, respectively. As shown in FIG. 1A, there is an empty, unused space 26 located between an outer diameter 27 of the center shield component 24 and the inner diameter 23 of the insulating tube 12 and therefore, the wall thickness of the center shield component 24 is not maximized. As a result, when subjected to interruption duties of a high number of shots of a high current or long arcing duration, as in the case of an asymmetrical current, the center shield wall is easily burned through.
Generally, an electrically floating center shield assembly is secured to the vacuum interrupter envelope via a center flange that is more susceptible to being braze-joined to or otherwise securely positioned with the insulating ceramic casing of the vacuum interrupter envelope. The cylindrical center shield assembly is slid into the ring-shaped flange opening. The maximum outer diameter (OD) of the center shield component is thus limited by the internal diameter (ID) of the center flange. The maximum OD of the center shield component is typically no more than a few thousands of an inch larger—for press fitting—than the smallest value of the ID of the center flange. This, in turn, limits the maximum diameter of the contacts that can be fitted inside the center shield component. As the diameter of the contacts is increased, there is a greater risk of burning through the shield wall due to a number of fault currents of a high amplitude.
There is known a vacuum interrupter and Cu—Cr alloy-based center shield design, wherein the maximum OD of the center shield component is larger than the ID of the opening of the center flange (e.g., snap-ring, in a particular embodiment). However, the thicker portion of the Cu—Cr shield wall is not employed to maximize the capability of the center shield component to withstand arc erosion because the contact gap is not aligned entirely with the thickest portion of the center shield wall. Instead, the thickest portion of the center shield wall is used for the purpose of creating a large enough step to secure the relatively heavy center shield to the center flange.
FIG. 2 is a cross-section view of a vacuum interrupter 10′ in accordance with the prior art. FIG. 2 includes the vacuum envelope 50 consisting of the insulating tube 12 and the end seals 51 and 52, the arc-enduring Cu—Cr alloy-based center shield component 24 and the opposing metal end components 13, 15 (which form the center shield assembly), the overlaps 37 and 38, the first electrode assembly 20, the second electrode assembly 22, the bellows 28, the bellows shield 48, the first electrode contact 30, the first terminal post 31, the first vapor shield 32, the second electrode contact 34, the second terminal post 35, the second vapor shield 36, and the end shield 58 as shown in FIG. 1. In addition, the vacuum interrupter 10′ also includes a center flange in the form of a snap-ring 25A (as shown in FIG. 2A) that is used to secure the arc-enduring Cu—Cr alloy-based center shield component to the insulating tube 12.
FIG. 2A is a detail view of the vacuum interrupter 10′ as shown in FIG. 2, when the vacuum interrupter 10′ is in the open position, with the contact gap 14 formed between the first and second electrode assemblies 20,22. As shown in FIG. 2A, there is no empty, unused space (26 as shown in FIG. 1A) between the outer diameter 27 of the center shield component 24 and the inner diameter 23 of the insulating tube 12. In contrast to FIG. 1A, FIG. 2A shows that a portion of the shield wall 29 has a maximum thickness. This portion of the shield wall 29 is created as a geometric step for securing the snap-ring flange 25A. The contact gap 14 is not positioned such that it is entirely in alignment with the shield wall 29 having a maximum thickness and outer diameter. As a result, when subjected to interruption duties of a high number of shots of a high current or long arcing duration, as in the case of an asymmetrical current, the center shield wall is easily burned through at the location where the wall thickness is not maximized.
FIG. 3 is a cross-section view of another vacuum interrupter 10″ in accordance with the prior art. FIG. 3 includes the vacuum envelope 50 consisting of the insulating tube 12 and end seals 51 and 52, first electrode assembly 20, second electrode assembly 22, bellows 28, bellows shield 48, first electrode contact 30, first terminal post 31, second electrode contact 34, and second terminal post 35, as shown in FIGS. 1 and 2. As shown in FIG. 3, the vacuum interrupter 10″ includes a center shield component 24A, which is secured to the insulating body 12 via a ledge on its internal (ID) wall. The rather complex shape of the center shield component 24A needed for such a mounting mechanism requires that it be made of a material that is not an arc-enduring Cu—Cr alloy-based material. For example, the center shield component 24A can be composed of a material that is more formable than an arc-enduring Cu—Cr alloy-based material, such as, but not limited to, pure copper or stainless steel.
FIG. 3A is a detail view of the vacuum interrupter 10″ and non-arc-enduring (e.g., non-Cu—Cr alloy-based) center shield component 24A, as shown in FIG. 3, when the vacuum interrupter 10″ is in the open position, with the contact gap 14 formed between the first and second electrode assemblies 20, 22. The mechanism for securing the center shield component 24A to the vacuum envelope 50 results in a shield wall 40 having a uniform thickness, e.g., there are no overlap locations to join a metal end to a non-metal end (of the Cu—Cr alloy-based center shield component), as shown in FIGS. 1A and 2A. That is, there are no overlaps 37, 38 (as shown in FIGS. 1A and 2A), each of which overlap a Cu—Cr alloy-based center shield wall. Thus, as shown in FIG. 3A, there is no thickness variation such that one portion of the shield wall can have a greater thickness than another portion of the shield wall. Such a shield made with a non-arc-enduring material serves solely the purpose of shielding the insulating tube 12, and does not actively participate in the arcing duty. When accidentally hit by the arcing in between the opening contacts, such a shield either melts excessively, in the case of a copper shield, or re-solidifies into dielectrically detrimental pointy features as in the case of a stainless steel shield. As a result, they have to be placed a significant distance (relatively far away) from the contact gap. In other words, only a relatively small diameter of the contacts can be employed for any given diameter of the center shield.
There is room for improvement in the design and manufacture of vacuum interrupters employing a center shield component composed of Cu—Cr alloy-based material, with or without additional minority alloying element or elements. It is an object of the disclosed concept to develop vacuum interrupters employing a floating center shield component composed of Cu—Cr alloy-based material, wherein the contact assemblies are axially positioned within the vacuum envelope such that the contact gap axial position is in alignment with a portion of the wall of the center shield component having a maximum thickness.